The Fischer-Tropsch process can be used for the conversion of synthesis gas (from hydrocarbonaceous feed stocks) into liquid and/or solid hydrocarbons. Generally, the feed stock (e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass) is converted in a first step into a mixture of hydrogen and carbon monoxide; this mixture is often referred to as synthesis gas or syngas. The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds and water. The obtained paraffinic compounds range from methane to high molecular weight hydrocarbons. The obtained high molecular weight hydrocarbons can comprise up to 200 carbon atoms, or, under particular circumstances, even more carbon atoms.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidized bed reactors, such as entrained fluidized bed reactors and fixed fluidized bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
During a Fischer-Tropsch process paraffinic compounds of different weights are formed. Additionally water is formed. When product is withdrawn from a Fischer-Tropsch system in most processes some unconverted syngas leaves the system as well. Further, gaseous components such as carbon dioxide, nitrogen and argon may leave the system when product is withdrawn from a Fischer-Tropsch system. Gaseous components such as carbon dioxide, nitrogen and argon may, for example, be present in the syngas used for the Fischer-Tropsch reaction.
Often some off-gas is removed from a Fischer-Tropsch process, regardless the number of recycles and regardless the number of Fischer-Tropsch reactors in the Fischer-Tropsch process. All or a part of the removed off-gas may be used as fuel, for example as fuel for furnaces. It is desired that at least a part of the valuable C3+ hydrocarbons is removed from the Fischer-Tropsch off-gas before the off-gas is used as fuel or before it is disposed of.
US 2011/0306682 A1 discloses a method for recovering hydrocarbon compounds from a gaseous by-product of a Fischer-Tropsch synthesis reaction. An absorption solvent including liquid hydrocarbons is used to absorb hydrocarbon compounds. A carbon dioxide gas absorbent is used to remove the carbon dioxide gas. The carbon dioxide gas absorbent may contain amines such as alkanol-amines. The gaseous by-product is fed to an absorber (112) comprising a mixture of the carbon dioxide gas absorbent and the liquid hydrocarbon compounds. The CO2 absorbent (with the absorbed CO2) and the hydrocarbon compounds are separated in a separator (115). The CO2 is stripped off using heat (114), and the CO2 absorbent is reused. The absorbed light hydrocarbons are recovered (116) and the liquid hydrocarbons are reused.
Disadvantages of the process disclosed in US 2011/0306682 A1 are that two absorbers are required, heat and thus energy are required to strip off the CO2 from the absorbent, and the line-up is relatively complex.
An object of the present invention is to recover hydrocarbons and hydrogen from Fischer-Tropsch off-gas. The present invention particularly aims at the recovery, or removal, of C3+ hydrocarbons, and optionally also C3+ oxygenates, from Fischer-Tropsch off-gas. The present invention further aims at an optimal processing of the off-gas to recover hydrogen. Preferably the process is energy-efficient, and preferably the process is performed in a relatively simple line-up.
The present invention particularly aims at the recovery of C3+ hydrocarbons and hydrogen from Fischer-Tropsch off-gas comprising hydrogen, carbon monoxide, carbon dioxide, nitrogen, methane, ethane and C3+ hydrocarbons. The off-gas may additionally comprise other components such as argon. The gas may comprise oxygenates. In case the off-gas comprises C3+ oxygenates, C3+ oxygenates may also be removed with the process of the present invention.